Controlled reduction of silver halide grains formed during precipitation

ABSTRACT

The preparation of a silver halide light-sensitive emulsion is improved by creating reduction reaction conditions during precipitation of the silver halide grains at a time prior to the end of physical ripening so as to form reduction centers. But the creation of the reduction centers is controlled to prevent production of nuclei which result in spontaneous development.

United States Patent [1 1 Claes et a1.

1 July 1,1975

[ CONTROLLED REDUCTION OF SILVER HALIDE GRAINS FORMED DURING PRECIPITATION [75] Inventors: Frans Henri Claes, Edegem,

Belgium; Eberhard Klein, Opladen, Germany; Marcel Jan Libeer, Mortsel, Belgium; Erik Moisar, Cologne, Germany [73] Assignee: Agfa-Gevaert Aktiengesellschaft,

Leverkusen-Bayerwerk, Germany 22 Filed: Jan. 23, 1973 21 Appl. No.: 326,126

[30] Foreign Application Priority Data [58] Field of Search 96/107, 94 R, 108

[5 6] References Cited UNITED STATES PATENTS 2,518,698 11/1948 Lowe et al. 96/107 2,666,700 1/1954 Baldsiefen 3,458,316 7/1969 Viro 96/1 14.7 3,679,424 7/1972 Barbier et a1. 96/107 Primary ExaminerWon H. Louie, Jr. Attorney, Agent, or Firm-Connolly and Hutz ABSTRACT 9 Claims, N0 Drawings 1 CONTROLLED REDUCTION OF SILVER HALIDE GRAINS FORMED DURING PRECIPITATION This invention relates to a process for the preparation of highly light-sensitive silver halide emulsions and to photographic materials containing these emulsions.

It is known that light sensitive silver halide grains of photographic emulsions manifest relatively little sensitivity to light immediately after precipitation or even after physical ripening, known as Ostwald ripening.

Sensitivity centres are formed on the surfaces of these crystals in the course of chemical ripening, which substantially improves the sensitivity to light and in some cases also the gradation.

On exposure to light, latent image nuclei which are capable of development are formed at or in the vicinity of these sensitivity centres, their position therefore depending on the position of the centres formed in the course of chemical ripening. If the sensitivity centres are situated in the interior of the silver halide particles, predominantly internal latent image nuclei are formed on exposure, but if the sensitivity centres are situated at the surfaces of the silver halide particles, then the latent image nuclei will be formed mainly at the surfaces. The relative positions of the sensitivity centres and hence the relative positions of the latent image nuclei formed on development determines the nature of the developer required for the development of the exposed material. For so-called internal latent images, it is necessary to use so-called internal developers, that is to say developers which contain a solvent for the silver halide, whereas emulsions which contain the latent image nuclei on the surfaces of the silver halide particles can be developed with conventional developers.

Upon chemical ripening, which results in the formation of sensitivity centres which, as far as is at present known consist of silver sulfide or noble metal atoms, in particular gold atoms, highly light-sensitive silver halide emulsions are obtained. Since there is a constant demand in practice for increasingly light-sensitive silver halide emulsions there was also a demand for methods for the production of more highly sensitive silver halide emulsions.

It is among the objects of the present invention to produce silver halide emulsions with maximum sensitivity.

We now have found a process for the production of photographic silver halide emulsion by precipitation, physical ripening and optionally by chemical ripening in which a reducing agent is added during precipitation of the silver halide or before or during physical ripening and reduction is carried out under such conditions as to prevent the formation of any fog centers which would be liable to undergo spontaneous development when a layer of this emulsions (the emulsion which is obtained after reduction treatment and prior to completion of crystal growth to the final size) which has a silver halide content which corresponds to 3 g of silver nitrate per m is developed for 4 minutes at C with a surface developer solution of the following composition:

p-Monomethylaminophenol sulfate 1,5 g Sodium sulfite (anhydrous) 25,0 g Hydroquinone 6.0 g Sodium carbonate (anhydrous) 40.0 g Potassium bromide 1.0 g

Water up to 1000 ml The silver halide emulsions obtained by the process according to the invention have maximum sensitivity to light, form latent image nuclei at the surface when exposed to light and can be developed with the usual surface developers.

The emulsions obtained by the process according to the invention have unexpected properties in that the previous rules concerning the topographic association of the sensitivity centres with the latent image nuclei do not apply to them. When these emulsions are exposed to light, latent image nuclei are formed at the surface and not in the interior of the grain at the centres produced by reduction. The essential feature of the process of this invention is that the silver halide grains are produced under reducing conditions before they have reached their final size. The fog produced in the course of this reduction must not be capable of spontaneous development and its density should not exceed 0.2.

The silver halide grains in the emulsion according to the invention are modified at a certain depth or at various stepwise variable depths by the reduction, presumably due to the formation of centres of silver formed upon reduction. These centres do not react in the same way as the usual sensitivity centres since they do not promote the formation of latent image nuclei in the interior of the grain on exposure to light.

Apart from their improved sensitivity to light, the silver halide emulsions prepared by the process according to the invention have a lower gradation than a corresponding silver halide emulsion which has a similar particle size distribution but has been prepared without the intermediate reducing reaction.

The nature of the reducing treatment and the exact stage at which it is carried out in the course of precipitation or physical ripening, determine whether the predominant effort achieved is an increase in sensitivity or a lowering in the gamma value.

The earlier the reducing agent is added during the formation of the silver halide grain, i.e., the deeper down in the interior are the reduction centres produced, the more powerfully will the gradation be reduced. If the reducing agents are added at a later stage of production of the silver halide, then the sensitivity increasing effect predominates and the gradation is only slightly reduced, if at all. The reducing agent influences neither the grain size nor the grain size distribution.

The quantity of reducing agent used must not be so large that the silver nuclei formed by reduction will be capable of development by light as in print-out materials. The reducing agents according to the invention are preferably not be used together with compounds which promote the printout effect such as, for example the sulfur compounds used for print-out materials according to U.S. Pat. No. 1,623,499. In principle, any inorganic or organic substance which acts as reducing agent for silver halides may be used, provided that it does not contain any labile sulfur atoms such as compounds of the mercapto, thio or thiosulfate series.

Suitable reducing agents are e.g. hydrazine which may be in the form of the hydrate, hydrazine derivatives ascorbic acid, hydroquinone or formamidine sulfinic acid (thiourea dioxide). It is preferred to use inorganic reducing agents e. g. tin-(Il)-chloride or the above mentioned thiourea dioxide. The quantity of reducing agent added may vary within wide limits and depends on the nature of the reducing agent and of the silver halide and on the effect desired. Even a very small quantity of reducing agent causes a substantial increase in the sensitivity and a reduction in the gradation. The quantity of reducing agent used according to this invention should generally not exceed 0.75 X milliequivalents per g of silver ions. Quantities of 0.1 to 10 mg per kg of silver nitrate have in most cases been found to be sufficient. In the case of thiourea dioxide it is sufficient to use for example 0.5 to 7.0 mg per kg of silver nitrate.

To provide the reducing conditions necessary for the process according to the invention it is not essential to use a reducing agent. It is sufficient to provide the conditions employed in the so-called Silver Digestion technique. Methods of employing this technique have been described in the publication by H. W. Wood in the J. phot. Sci. 1 (1953), 163. The Silver Digestion technique is performed at slightly elevated temperatures of about 50C and may be carried out at elevated pH values of between 7 and 10. The silver ion concentration is increased, i.e., the method is carried out at a relatively low pAg value of between about 7 and 0, preferably about 3.

The precipitation of the silver halide is carried out in known manner, either by adding one of the components for precipitation to the other component or by simultaneously running both precipitation components into a solution of the protective colloid (double jet method).

The first of these methods is preferably carried out by adding an aqueous solution of a soluble silver salt, e.g. silver nitrate, with stirring, into an aqueous alkali metal halide solution which contains the protective colloid, preferably gelatin and may also contain other additives. If the method of the simultaneous addition of both components is employed, an aqueous solution of a soluble silver salt and a solution of a water-soluble alkali metal ahlide are simultaneously added to a solution of the protective colloid with stirring.

If desired, the reducing agent may be supplied in the apparatus containing the solution of the protective colloid or it may be contained in the solution of the alkali metal salt which is to be added. The desired effect according to the invention is not obtained if the reducing agent is introduced into the solution of the soluble silver salt because in that case the reducing agent may be used up before the formation of silver halide crystals begins.

According to a preferred method of performing the present invention, precipitation by the method of the simultaneous addition of both components is interrupted when the particles have reached a certain size. The silver halide particles already formed are then treated in the emulsion with a reducing agent at a concentration sufficiently low to prevent the formation of fog nuclei capable of spontaneous development. Any excess of reducing agent may then be removed and precipitation continued so that the silver nuclei formed by decomposition and reduction are then enclosed in the silver halide grains at certain depths in the layer.

The brief reducing reaction carried out as the intermediate treatment may be performed similarly when employing the method of providing one precipitation component in the apparatus to which the other is then added. In this method, an aqueous silver nitrate solution is introduced into a stoichiometric excess of aqueous alkali metal halide solution containing the protective colloid. When the required particle size has been reached, the excess of alkali metal halide may be removed by washing. The reducing treatment is then carried out as described above and the process of crystal growth is continued by continuing the precipitation or by a simple physical ripening after the addition of a further quantity of akali metal halides so that any microcrystals formed are dissolved and the growth of larger crystals is promoted.

According to another method of the process, the reducing agent is added in a separate solution in water, a water-miscible organic solvent or a mixture of water and water-miscible solvents during the precipitation of the silver halide crystals. Suitable water-miscible solvents are: Lower alcohols such as methanol or ethanol, ketones such as acetone or methyl ethyl ketone and esters such as ethyl acetate or butyl acetate. This particular method of carrying out the present invention is distinguished by the ease of the controlled addition of the reducing agent during the formation of silver halide grains. The gradation characteristics of the silver halide emulsions can thereby be better controlled.

In all these various methods of performing the invention the reducing agent need not necessarily be added all at once and within a very short time, but may be interrupted as desired or continued over the whole period of precipitation.

The method according to the invention is suitable for the preparation of any silver halide emulsions including e.g. coarse grained and fine grained emulsions. Thus, for example, fine-grained silver halide emulsions with a relatively flat gradation and narrow particle size distribution can be produced by the process of the invention. Such emulsions contain silver halide grains with a diameter of less than 0.6 p. and preferably 0.05 to 0.3 ;L. A narrow particle size distribution means that at least percent by weight and/or at least 95 percent of the number of silver halide grains have a diameter which deviates by not more than 40 percent and preferably not more than 30 percent from the average particle diameter. The particle diameter and average particle diameter can be determined, for example, by the methods described in Methods of Particle Size Analysis by R. P. Loveland, ASTM Symposium on Light Microscopy, 1952, STP 143,94.

Such fine-grained silver halide emulsions are preferably prepared by the double jet method at certain pH and pAg values. The precipitation temperature employed is normally 30 to 60C at pH values of up to 9, preferably up to 6, and pAg values of up to about 9.8. Reference may be made to the publication Properties of Photographic Emulsion Grains by E. Klein and E. Moisar, J. Pho. Science, 12, (1964), 242 to 251. Value is meant the negative common logarithms of the Ag"- concentration.

The process of this invention may be employed for the preparation of any kind of silver halide emulsions, e.g. silver chloride emulsions, silver bromide emulsions, silver chlorobromide emulsions, silver iodobromide and silver iodochlorobromide emulsions. A particularly advantageous effect is achieved with silver bromide or silver iodobromide emulsions.

The process according to the invention may also be employed before the preparation of highly sensitive direct positive silver halide emulsions with flat gradations. These direct positive emulsions are prepared by surface fogging of the silver halide grains with a reducing agent or by light, e.g. in accordance with British Pat. No. 1,555,404.

The process according to the invention may also be employed for the preparation of emulsions with a composite grain structure. The nucleus of the silver halide grain may be chemically sensitized and the shell fogged by the usual methods to obtain an emulsion of the direct positive type. Reference may be made in this connection to British Pat. No. 1,027,146 or US. Pat. No. 3,367,778. If the process according to the invention is used for the preparation of emulsions which have a composite grain structure, in particular if the shell is formed by precipitation on the nuclleus, direct positive emulsions with a relatively flat gradation are obtained. Subsequent processing of the emulsions is carried out in the usual manner.

After the reducing treatment in accordance with the process of this invention, the silver halide emulsions are physically ripened in the usual manner and solidified by cooling or precipitated by flocculation. The excess salts are then washed out and if desired the emulsions may be chemically ripened by known methods. The usual additives may then be added to the finished casting solution.

The process according to the invention may be em ployed for producing any silver halide emulsions. Suitable silver halides are silver chloride, silver bromide or mixtures thereof, optionally with a small silver iodide content of up to mols percent. The silver halides may be dispersed in the usual hydrophilic binders, for example in carboxymethylcellulose, polyvinyl alcohol, polyvinyl pyrrolidone, alginic acid and its salts, esters or amides or proteins, preferably gelatin.

The emulsions may also be chemically sensitized, e.g. with reducing agents such as tin-ll salts, polyamines such as diethylene triamine or sulfur compounds as described in US. Pat. No. 1,574,944 or in the book by Mees entitled Theory of the Photographic Process (1954), pages 149 to 161. Chemical sensitization of the given emulsions may also be carried out using salts of noble metals such as ruthenium, rhodium, palladium, iridium, platinum or gold as described in the article by R. Koslowsky, Z. Wiss. Phot., 46, 65-72 (1951). Compounds of the thiomorpholine series, e.g. those described in French Pat. No. 1,506,230 or polyalkylene oxides, particularly polyethylene oxide and derivatives thereof, are also suitable for this purpose.

The emulsions may also be optically sensitized, e.g. with the usual polymethine dyes such as neutrocyanines, basic or acid carbocyanines, merocyanines or rhodacyanines, hemicyanines, styryl dyes, oxonoles and the like. Sensitizers of this kind have been described in the work by F. M. Hamer entitled The Cyanine Dyes and related Compounds (1964), Interscience Publishers a division of John Wiley & Sons, New York.

The emulsions may be hardened in the usual mannner, for example with formaldehyde or halogenated aldehydes which contain a carboxyl group, such as mucobromic acid diketones, methane sulfonic acid esters, dialdehydes and the like.

Emulsions prepared by the process according to the invention may be used for various photographic materials e.g. phototechnical films for copying materials or original photographic materials and particularly for high sensitivity photographic materials. They may also be used for X-ray films, for color photographic materials of all kinds e.g. for light sensitive emulsions which contain color couplers for materials used in the silver dye bleaching process and for photographic materials used for the silver salt diffusion process or any other instant image processes.

EXAMPLE 1 Preparation of a silver chloride emulsion:

A molar solution of silver nitrate is added at a constant rate ml/min) to l l of an aqueous solution containing 50 g of inert gelatin. At the same time, a sodium chloride solution is added at such a rate that the pAg value ramains constant (+40 mV). The pAg is measured using a saturated calomel electrode against a silver electrode. The temperature was maintained constant at 50C. The time required for precipitation was 13 minutes.

In tests A, B, C and D, a 0,1 percent aqueous thiourea dioxide solution was added at the beginning of precipitation, at the 4th minute at the 8th minute and at the 12th minute of precipitation. The concentration of thiourea dioxide was 1.25 mg/kg of silver nitrate. 1n test E one quarter of the quantity of thiourea dioxide given above was added at the beginning of precipitation and after the fourth, eighth and twelfth minutes.

The average particle size of the silver chloride crystals obtained in precipitation tests A, B, C, D and E was 0.2 ,um. The emulsion was flocculated with ammonium sulfate after precipitation and decanted. Finally it was chemically ripened in the presence of a compound which contained labile sulfur and had the following structural formula:

p-Monornethylaminophenolsulfate l .5 Sodium sulfite (anhydrous) 25 Hydroquinone 6 Sodium carbonate (anhydrous) 40 Potassium bromide 1 Water up to 1000 l The sensitometric results of these samples are summarized in Table I below. The value ('y) is the 'y-value in the straight lined portion of the density curve. The value (yt is the value of the threshold of the characteristic curve, measured at a density of 0.1 above the fog. The sensitivity (S) is expressed in relative logarithmic E values (log I.t). The lower this value, the higher is the sensitivity of the emulsion obtained.

Table I Sample Fog (7') (7 Comparison 0.09 2.05 1.30 4.7 A 0.07 1.30 1.10 5.4

EXAMPLE 2 A silver bromide emulsion was prepared by the double jet method at a constant temperature of 60C by the simultaneous addition of a molar aqueous silver nitrate solution and a 1.5 molar aqueous potassium bromide solution to 1 l of an aqueous 2 percent gelatine solution. The bromide excess was controlled to maintain a pAg value of 0 mV measured with a calomel electrode against a silver electroce.

The silver nitrate solution was added at the rate of 57 ml per minute. Precipitation took 13 minutes. 1.25 mg of thiourea dioxide per kg of silver nitrate were added continuously in the form of a 0.1 percent solution in the course of precipitation.

In test A, B and C, the addition of thiourea dioxide solution took place at the 4th, 8th, and 12th minute of precipitation. In another test D, one quarter of the total quantity of thiourea dioxide solution was added at the beginning of precipitation and again after the 4th, 8th and 12th minute.

The average particle size of the silver halide crystals was 0.2.

After precipitation, the emulsions were flocculated with ammonium sulfate and washed, the flocculated silver halide emulsions were chemically ripened by the addition of Na;, [Au 0 M (9 mg per mol of silver bromide) and the resulting emulsions were applied to a polyethylene terephthalate support at a concentration of 2 g of silver in the form of silver halide per m The coated samples were exposed behind a neutral grey wedge (0.2 wedge constant) for 0.05 seconds with a 750 Watt tungsten lamp at a distance of 40 cm and developed in the same developer as in Example 1.

The sensitometric results of these samples are shown in Table 11 below compared with the results obtained with a sample of emulsion prepared without the addition of thiourea dioxide but in otherwise identical manner.

A silver bromide gelatin emulsion was prepared by simultaneous addition of a molar aqueous silver nitrate solution and a 1.5 molar aqueous potassium bromide solution to 1 1 of 2 percent aqueous gelatin solution at 60C, the pAg being kept constant at 0 mV and the pH at 6.5 during precipitation. Precipitation took 13 minutes. The silver nitrate solution was added at the rate of 57 ml per minute. The average particle size of the resulting silver bromide grains was 0.2.

Comparison tests were carried out with the addition of 0.75, 1,25 and 2 mg of thiourea dioxide per kg of silver nitrate. The thiourea dioxide solution was added in the form of a 0.1 percent aqueous solution. In test A the thiourea dioxide solution was added immediately at the beginning of precipitation (minute 0) and in tests B and C it was added at the fourth and twelfth minute of precipitation.

When the emulsion had been washed, the individual samples were chemically ripened as described in Example 2 and coated on a support of cellulose triacetate to a thickness corresponding to a silver halide concentration of 2.1 g of silver in the form of silver halide per m and the samples were then exposed and developed as described in Example 2. The results of the sensitometric tests which are defined in the same way as described in Example 1 are summarized in Table III below.

The experiment described in Example 3 was repeated with the difference that precipitation was carried out at pAg mV and pH 6. The silver bromide crystals obtained have an average particle size of 0.4. The quantity of thiourea dioxide added was 0.75 mg/kg of silver nitrate.

In test A l the thiourea dioxide was added immediately at the beginning (minute 0) and in test B 1 it was added after the average particle size of the silver halide crystals had reached a value of 0.2 pm. The emulsion samples were washed and then chemically ripened as described in Example 2.

The amount of silver applied varied in the different layers of emulsion. The values are shown in column 5 of Table IV in terms of g of silver in the form of silver halide per m The sensitometric results were interpreted in the same way as in Example 1. The results are summarized in the following Table.

EXAMPLE The tests described in Example 3 were repeated with the difference that silver iodobromide emulsions (3 mols percent of Agl) were prepared instead of silver bromide emulsions.

0.75 mg of thiourea dioxide per kg of silver halide was added to sample A, B and C, the addition to sample A being carried out at the beginning of precipitation, the addition to sample B in the 4th minute of precipitation and to sample C in the 12th minute.

The amount of silver applied in the emulsion layers is entered in the last column of the following table in terms of g of silver in the form of silver halide per m The results os sensitometric interpretation are summarized in Table V.

A direct positive silver halide emulsion with a composite grain structure was prepared as follows:

A 3-molar aqueous silver nitrate solution and 3- molar aqueous potassium bromide solution were simultaneously added to a solution of 50 g of gelatin in 1300 ml of water. The reaktion temperature was 60C and the pAg was kept constant at 0 mV and the pH at 6.5 in the course of precipitation. Precipitation took 7 minutes, the silver nitrate solution being added at the rate of 19 ml/min. The average particle size of the grains obtained was 0.13 am.

The silver halide grains obtained were then ripened in the conventional manner by the addition of 3.5 ml of a 10 molar aqueous solution of thiourea dioxide, based on the total quantity of crystals formed, and 30 minutes heating at 45C. 1.5 ml of an aqueous 0.08 percent solution of gold-(IlI)-chloride and 1.5 ml of an aqueous 2 percent solution of ammonium thiocyanate are then added. Following the additon of these components, the reaction mixture was heated for a further 10 minutes at 45C. A comparison sample with a conventional structure was then produced by precipitating the shell as follows (2nd precipitation):

An aqueous solution of ammonia was added to adjust the pH to 9.3 and an aqueous 3-molar potassium bromide solution and an aqueous 3-molar silver nitrate solution were then added simultaneously at a rate of 19 ml/min for 35 minutes. The pAg was kept constant at +20 mV (measured on a silver electrode against the calomel electrode).

To prepare emulsion A according to the invention the second precipitation was carried out as follows: The so-called silver digestion method was employed by adjusting the pAg to +150 mV (silver/saturated calomel electrode) during the second precipitation. The precipitation temperature of the comparison emulsion and of emulsion A was 50C.

Both emulsions were then precipitated after the addition of 84 g of gelatin and then washed in the usual manner.

These two emulsions which both had a composite grain structure were then treated for minutes at 45C with 11 ml of a 10 molar aqueous solution of thiourea dioxide per kg of emulsion containing 1 10 g of silver bromide. A fog which is capable of spontaneous development was thereby formed on the surface of the grains.

After fogging, each of the sample was treated with the same quantity of gelatin and the pAg was adjusted to 20 mV (silver/saturated calomel electrode) by the addition of an aqueous potassium bromide solution.

The two samples of emulsions each contain 1 10 g of silver bromide, 80 g of gelatin and 1 g of saponin as wetting agent per kg of emulsion. 500 mg of formaldehyde were added as hardener shortly before coating. The emulsion samples were then cast on a polyethylene terephthalate support and dried. The amount of silver applied was 7.8 g of silver bromide per m After drying the samples were exposed to an incandescent lamp behind a neutral stop wedge (wedge constant 0.20 and developed with a surface developer of the following composition at 20C (development time 135 seconds):

Sodium sulfite (anhydrous) 3 Trioxymethylene Potassium metabisulfite Boric acid (crystalline) Hydroquinone 2 Potassium bromide Polyethylene oxide (average molecular weight 2000) Water 5 up to 1000 a: rcnonornnarm The sensitometric results obtained for the two samples are summarized in Table VI.

The starting emulsion used for the remaining tests was prepared by running 1,800 ml of a 3M silver nitrate solution and 1,800 m1 of a 3M potassium bromide solution into a solution of g of inert gelatin in 2,000 ml of water at 50C over a period of 3 hours. The rate of addition was controlled electrometrically so that the concentration silver ions in the solution phase corresponds to a pAg of about 8. An additional 80 g of gelatin were added after precipitation and the emulsion was then solidified, washed and finally adjusted with potssium bromide to a pAg =9.

Silver nitrate solution was added to 400 g of this emulsion at 50C and pH 6.5 to 7 until the pAg in the solution phase was 3. After 10 minutes, additional silver bromide was precipitated on the grains of the homodisperse original emulsion by simultaneous addition of 600 ml of 3M silver nitrate solution and 600 ml of 3M potassium bromide solution the pAg, which was 3 at the beginning of this surface precipitation, being gradually adjusted to 9. After the addition of 60 g of gelatin, the emulsion was washed and other wise treated in the usual manner. The resulting emulsion is also homodisperse (narrow grain size distribution). The silver bromide crystals have a cubical structure and a particle size of about 0.3 pm. The emulsion was chemically ripened with Na -,[Au(S O 2 ml of molar solution per 200 g of emulsion. After optimum ripening, the emulsion was cast on a cellulose acetate support, exposed and developed. The density curve shows that the sensitivity obtained is higher by 0.7 log units than that of an optimally ripened comparison emulsion with the same particle size which was prepared in the same way except that the adjustment to a pAg of 3 was omitted.

EXAMPLE 8 2 ml of a 3 percent hydrazine hydrate solution were added to 400 ml of the starting emulsion described in Example 7. After 60 minutes treatment at 50C, precipitation was continued as described in Example 7 by simultaneous addition of 600 ml of 3M silver nitrate and 600 ml of 3M potassium bromide solution. A comparison emulsion was prepared in a similar manner but without the addition of hydrazine. Even without chemical ripening, treatment with hydrazine during precipitation was found to result in an increase in sensitivity by 0.3 log I.t units. After optimum ripening with a gold compound as described in Example 7, the emulsion treated with hydrazine in accordance with the invention is more sensitive than the comparison emulsion by 0.35 log l.t units.

EXAMPLE 9 A starting emulsion was prepared as described in Example 7 except that a mixture of 94 mols percent potassium bromide and 6 mols percent of potassium iodide was used for precipitation instead of potassium bromide.

400 ml of this silver iodobromide emulsion were digested for 60 minutes at 50C with the addition of 6 ml of 3 percent hydrazine hydrate solution. Further precipitation onto the grains thus formed was performed using the mixture of potassium bromide and potassium iodide as described in Example 8. After optimum ripening with gold, the sensitivity was found to be increased by 0.5 log units compared with that of a comparison emulsion which is identical except that the treatment with hydrazine was omitted.

EXAMPLE 10 A heterodisperse starting emulsion with a relatively wide particle size distribution was prepared by adding 100 ml of a solution containing 400 g of silver nitrate to a solution of 32 g of potassium bromide and 240 g of inert gelatin in 4,000 ml of water in the course of 30 seconds at 68C. The emulsion was then left to solidiify without physical ripening, washed and remelted. An additional 240 g of gelatin was then added. The pAg was adjusted to about 9.

650 g of this emulsion used as starting material were adjusted to pAg 3 with silver nitrate solution. After leaving the emulsion to digest for 10 minutes at 50C, 470 ml of a 3M silver nitrate solution and 470 ml of a 3M potassium bromide solution were introduced by the double jet method in the course of 100 minutes, the original pAg value of about 3 gradually increasing in the course of precipitation to about 10. After precipitation, 50 g of gelatin were added and the emulsion was solidified and washed in water in the usual manner. The emulsion was'heterodisperse and contains twinned silver bromide crystals with an average grain size of about 0.7 ,um. It was optimally ripened with 2 ml of a 10' molar solution of Na [Au(S 0 per 200 g of emulsion, cast to form a film on a polyethylene terephthalate support, exposed and developed. Compared with a comparison emulsion which had the same grain size and was otherwise similar except that it had been prepared without the adjustment to pAg 3 with silver nitrate the emulsion showed an increase in sensitivity by 0.3 log units.

EXAMPLE 1 l 650 g of the starting emulsion described in Example 10 were digested for minutes at 50C with an addition of 15 ml of a 3 percent hydrazine hydrate solution. Precipitation was then carried out as described in Example 10 using 470 ml of 3M silver nitrate solution and 470 ml of 3M potassium bromide solution and the emulsion was washed and ripened as described there. Compared with a comparison emulsion which was prepared without the treatment with hydrazine, the emulsion obtained showed an increase in sensitivity by 0.35 log units.

EXAMPLE 12 1500 g of the starting emulsion described in Example 10 were treated at 50C for 150 minutes with an addition of 40 ml of 3 percent hydrazine hydrate solution. After the addition of an aqueous solution of 12 g of potassium bromide it was physically ripened at 40C for 30 minutes. The bromide excess was removed by washing. The emulsion was then ripened with 1 ml of a W molar solution of Na [(S 0 per 200 g of emulsion. Compared with a comparison emulsion which was prepared in analogous manner except that the addition of hydrazine was omitted, the emulsion obtained showed an increase in sensitivity by 0.3 log units.

We claim:

1. In a process for the production of direct-positive photographic emulsions having silver halide grains of composite structure, the steps of first mixing components including a solution of a soluble halide salt and a soluble silver salt in the presence of an inert colloid to form a precipitate of a silver halide, chemically sensitizing the said silver halide and continuing precipitation to form an outer-shell of silver halide and finally fogging the surface of the outer-shell to form spontaneously developable fog nuclei wherein the improvement comprises precipitating the silver halide for the outershell at a pH of up to 9 and a pAg of up to 9.8 and subjecting the silver halide for the outer-shell during precipitation to a controlled reduction reaction by treatment with a reducing agent or light or by silver digestion to obtain silver halide grains in the silver halide emulsion of the outer-shell which silver halide grains contain silver nuclei which are not spontaneously developable without exposure to light upon treatment of a layer of said emulsion having a silver halide content corresponding to 3 g of silver nitrate per m with a surface developer of the following composition:

p-Monomethylaminophenol Sodium sulfite (anhydrous) -Continued Sodium carbonate (anhydrous) 40.0 g Water up to 1000 ml for a development time of 4 minutes and at a development temperature of 20C.

2. The process of claim 1, wherein reducing agents are added for the reducing treatment at the formation of the silver halide grains during precipitation.

3. The process of claim 2, wherein the reducing agent added is thiourea dioxide of hydrazine hydrate.

4. The process of claim 2, wherein the reducing agents are added in quantities of up to 0.75 X milliequivalents per g of silver ions.

5. The process of claim 1, wherein the reducing treatment is performed by adjusting the silver ion concentration to a high level with pAg values of between 7 and 0.

6. The process of claim 5, wherein thiourea dioxide is added in quantities of 0.5 to 0.7 mg per kg of silver nitrate.

7. A negative photographic emulsion of lightsensitive silver halide grains forming a latent image nuclei at the grain surface and not in the interior upon exposure to light produced by mixing components including a solution of a soluble halide salt and a soluble silver salt in the presence of an inert colloid to form a precipitate of a silver halide in an emulsion at a pH of up to 9 and a pAg of up to 9.8 and chemically ripening the silver halide and wherein during precipitation the silver halide is subjected to a controlled reduction reaction by treatment with a reducing agent or light or by silver digestion and the silver halide emulsion contains silver halide grains containing silver nuclei which are not spontaneously developable without exposure to light upon treatment of a layer of said emulsion having a silver halide content corresponding to 3 g of silver nitrate per in with a surface developer of the following composition:

p-Monomethylaminophenol sulfate 1.5 g Sodium sulfite (anhydrous) 25.0 g Hydroquinone 6.0 g

Sodium carbonate (anhydrous) 4 Water up to I000 ml produced by mixing components including a solution of a soluble halide salt and a soluble silver salt in the pres ence of an inert colloid to precipitate the silver halide of the outer-shell onto the silver halide of the said core at a pH of up to 9 and a pAg of up to 9.8 and finally fogging the surface of the outer-shell to form spontaneously developable fog and wherein during precipitation the silver halide of the outer-shell is subjected to a controlled reduction reaction by treatment with a reducing agent or light or by silver digestion to form within the silver halide of the outer-shell silver nuclei which are not spontaneously developable without exposure to light upon treatment of a layer of said emulsion having a silver halide content corresponding to 3 g of silver nitrate per m with a surface developer of the following composition:

Water up to I000 ml for a development time of 4 minutes and at a development temperature of 20C.

9. In a process for the preparation of negative photographic emulsions of light-sensitive silver halide grains forming a latent image nuclei at the grain surface and not in the interior upon exposure to light, the steps of first mixing components include a solution of a soluble halide salt and a soluble silver salt in the presence of an inert colloid to form a precipitate of a silver halide in an emulsion and then ripening the silver halide in the emulsion wherein the improvement comprises precipitation at a pH of up to 9 a pAg of up to 9.8 and subjecting the silver halide during precipitation to a controlled reduction reaction by treatment with a reducing agent or light or by silver digestion to obtain silver halide grains in a silver halide emulsion containing silver nuclei which are not spontaneously developable without exposure to light upon treatment of a layer of said emulsion having a silver halide content corresponding to 3 g of silver nitrate per in with a surface developer of the following composition:

Sodium carbonate (anhydrous) 4 Water up to 1000 ml for a development time of 4 minutes and at a development temperature 20C. 

1. IN A PROCESS FOR THE PRODUCTION OF DIRECT-POSITIVE PHOTO
 1. IN A PROCESS FOR THE PRODUCTION OF DIREC-POSITIVE PHOTO- 0 GRAPHIC EMULSIONS HAVING SILVER HALIDE GRAINS OF COMPOSITE STRUCTURE, THE STEPS OF FIRST MIXING COMPONENTS INCLUDING A SOLUTION OF A SOLUBLE HALIDE SALT AND A SOLUBLE SILVER SALT IN THE PRESENCE OF AN INERT COLLOID TO FORM A PRECIPITATE OF A SILVER HALIDE, CHEMICALLY SENSITIZING THE SAID SILVER HALIDE AND CONTINUING PRECIPITATION TO FORM AN OUTER-SHELL OF SILVER HALIDE AND FINALLY FOGGING THE SURFACE OF THE OUTER-SHELL TO FORM SPONTATEOUSLY DEVELOPABLE FOG NUCLEI WHEREIN THE IMPROVEMENT COMPRISES PRECIPITATING THE SILVER HALIDE FOR THE OUTER-SHELL AT A PH OF UP TO 9 AND A PAG OF UP TO 9.8 AND SUBJECTING THE SILVER HALIDE FOR THE OUTER-SHELL DURING PRECIPITATION TO A CONTROLLED REDUCTION REACTION BY TREATMENT WITH A REDUCING AGENT OR LIGHT OR BY SILVER DIGESTION TO OBTAIN SILVER HALIDE GRAINS IN THE SILVER HALIDE EMULSION OF THE OUTER-SHELL WHICH SILVER HALIDE GRAINS CONTAIN SILVER NUCLEI WHICH ARE SPONTANEOUSLY DEVELOPABLE WITHOUT EXPOSURE TO LIGHT UPON REATMENT OF A LAYER OF SAID EMULSION HAVING A SILVER HALIDE CONTENT CORRESPONDING 3 G OF SILVER NITRATE PER M2 WITH A SURFACE DEVELOPER OF THE FOLLOWING COMPOSITION:
 2. The process of claim 1, wherein reducing agents are added for the reducing treatment at the formation of the silver halide grains during precipitation.
 3. The process of claim 2, wherein the reducing agent added is thiourea dioxide of hydrazine hydrate.
 4. The process of claim 2, wherein the reducing agents are added in quantities of up to 0.75 X 10 2 milliequivalents per g of silver ions.
 5. The process of claim 1, wherein the reducing treatment is performed by adjusting the silver ion concentration to a high level with pAg values of between 7 and
 0. 6. The process of claim 5, wherein thiourea dioxide is added in quantities of 0.5 to 0.7 mg per kg of silver nitrate.
 7. A negative photographic emulsion of light-sensitive silver halide grains forming a latent image nuclei at the grain surface and not in the interior upon exposure to light produced by mixing components including a solution of a soluble halide salt and a soluble silver salt in the presence of an inert colloid to form a precipitate of a silver halide in an emulsion at a pH of up to 9 and a pAg of up to 9.8 and chemically ripening the silver halide and wherein during precipitation the silver halide is subjected to a controlled reduction reaction by treatment with a reducing agent or light or by silver digestion and the silver halide emulsion contains silver halide grains containing silver nuclei which are not spontaneously developable without exposure to light upon treatment of a layer of said emulsion having a silver halide content corresponding to 3 g of silver nitrate per m2 with a surface developer of the following composition:
 8. A DIRECT-POSITIVE PHOTOGRAPHIC SILVER HALIDE EMULSION WITH SILVER HALIDE GRAINS HAVING COMPOSITE GRAIN STRUCTURE AND CONSISTING OF A CORE OF A SILVER HALIDE WHICH IS CHEMICALLY SENSITIZED AND AN OUTER-SHELL OF A SILVER HALIDE THE SURFACE OF WHICH CONTAINS SPONTANEOUSLY DEVELOPABLE FOG WHEREIN THE SAID OUTER-SHELL IS PRODUCED BY MIXING COMPONENTS INCLUDING A SOLUTION OF A SOLUBLE HALIDE SALT AND A SOLUBLE SILVER SALT IN PRESENCE OF AN INERT COLLOID TO PRECIPITATE THE SILVER HALIDE OF THE OUTER-SHELL ONTO THE SILVER HALIDE OF THE SAID CORE AT A PH OF UP TO 9 AND A PAG OF UP TO 9.8 AND FINALLY FOGGING THE SURFACE OF THE OUTER-SHELL TO FORM SPONTANEOUSLY DEVELOPABLE FOG AND WHEREIN DURING PRECIPITATION THE SILVER HALIDE OF THE OUTER-SHELL IS SUBJECTED TO A CONTROLLED REDUCTION REACTION BY TREATMENT WITH A REDUCING AGENT OR LIGHT OR BY SILVER DIGESTION TO FORM WITHIN THE SILVER HALIDE OF THE OUTER-SHELL SILVER NUCLEI WHICH ARE NOT SPONTANEOUSLY DEVELOPABLE WITHOUT EXPOSURE TO LIGHT UPON TREATMENT OF A LAYER OF SAID EMULSION HAVING A SILVER HALIDE CONTENT CORRESPONDING TO 3 G OF SILVER NITRATE PER M2 WITH A SURFACE DEVELOPER OF THE FOLLOWING COMPOSITION:
 9. In a process for the preparation of negative photographic emulsions of light-sensitive silver halide grains forming a latent image nuclei at the grain surface and not in the interior upon exposure to light, the steps of first mixing components include a solution of a soluble halide salt and a soluble silver salt in the presence of an inert colloid to form a precipitate of a silver halide in an emulsion and then ripening the silver halide in the emulsion wherein the improvement comprises precipitation at a pH of up to 9 a pAg of up to 9.8 and subjecting the silver halide during precipitation to a controlled reduction reaction by treatment with a reducing agent or light or by silver digestion to obtain silver halide grains in a silver halide emulsion containing silver nuclei which are not spontaneously developable without exposure to light upon treatment of a layer of said emulsion having a silver halide content corresponding to 3 g of silver nitrate per m2 with a surface developer of the following composition: 